Fuel cells have been proposed as a power source for electric vehicles and other applications. One known fuel cell is the PEM (i.e., Proton Exchange Membrane) fuel cell that includes a so-called MEA (“membrane-electrode-assembly”) comprising a thin, solid polymer membrane-electrolyte having an anode on one face and a cathode on the opposite face. The anode and cathode typically comprise finely divided carbon particles, very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles, and proton conductive material intermingled with the catalytic and carbon particles. The MEA is sandwiched between a pair of electrically conductive contact elements which serve as current collectors for the anode and cathode, which may contain appropriate channels and openings therein for distributing the fuel cell's gaseous reactants (i.e., H2 and O2/air) over the surfaces of the respective anode and cathode.
Bipolar PEM fuel cells comprise a plurality of the MEAs stacked together in electrical series while being separated one from the next by an impermeable, electrically conductive contact element known as a bipolar plate or septum. The bipolar plate has two working surfaces, one confronting the anode of one cell and the other confronting the cathode on the next adjacent cell in the stack. The bipolar plate conducts electrical current between the adjacent cells. Contact elements at the ends of the stack contact only the end cells and are referred to as end plates.
Contact elements are often constructed from electrically conductive metal materials. In an H2 and O2/air PEM fuel cell environment, the bipolar plates and other contact elements (e.g., end plates) are in constant contact with highly acidic solutions (pH 3-5) and operate in a highly oxidizing environment, being polarized to a maximum of about +1 V (vs. the normal hydrogen electrode). On the cathode side the contact elements are exposed to pressurized air, and on the anode side exposed to hydrogen. Unfortunately, many metals are susceptible to corrosion in the hostile PEM fuel cell environment, and some also form highly electrically resistive, passivating oxide films on their surface (e.g., in the case of stainless steel, nickel, magnesium, aluminum, titanium, or alloys) that increases the internal resistance of the fuel cell and reduces its performance.
In light of the corrosion sensitivity of these metals, efforts have been made to develop conductive polymeric protective coatings. One effort includes protective polymeric coatings that have a minimum impact on electrical resistance and maintain an acceptable level of conductivity, however these coatings have the potential to peel or chip due to the potential reduction of coating adhesion while the element is exposed to the humidified gases and high temperature and pressure of the working fuel cell environment. Coating detachment could potentially expose the underlying metal substrate to corrosion and/or decreased conductivity.
Accordingly, there is a need for an increased adhesion of protective polymeric, and electrically conductive coatings to the substrate while maintaining electrical conductivity, to resist the fuel cell's hostile environment and to improve the overall efficiency, durability, and longevity of the electrochemical cell.